Wireless communication system and associated wireless communication method

ABSTRACT

A wireless communication system includes an access point and at least one station, wherein when the access point operates in a setup mode, both the access point and station confirm a specific time slot for transmitting a packet from the station to the access point; and when the access point operates in a data mode following the setup mode, the station directly uses the specific time slot determined in the setup mode to transmit the packet to the access point.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the priority of U.S. Provisional Application No.62/073,028, filed on Oct. 31, 2014, and U.S. Provisional Application No.62/180,038, filed on Jun. 15, 2015, which is included herein byreference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a wireless communication system, andmore particularly, to an access method between an access point and oneor more stations of the wireless communication system.

2. Description of the Prior Art

Many wired and wireless networks enable multiple access usingdistributed channel access mechanisms. In a typical instantiation, suchas that used in the IEEE 802.11 set of protocols, devices that shareaccess to the wireless medium, and that wish to transmit, willindependently choose a number of transmission-available time slots (a‘backoff’) to delay before attempting to transmit a packet. A newbackoff is chosen for each new packet. Each device's new backoff israndomly (or pseudo-randomly) chosen from a set of possible backoffsdetermined by the protocol. Different devices will (usually) choosedifferent backoffs (because the backoff choices are made independentlyon each transmitter), and then the devices will not attempt to transmitat the same time. In this way the protocol allows for different devicesthat implement the same underlying protocol to transmit at differenttimes, without any explicit sequence of negotiation or arbitrationmessages. This is a “distributed” coordination channel access mechanism.

However, two or more devices may choose the same backoff value. In thiscase a “collision” will occur.

For this distributed coordination system to work effectively, the numberof available backoff slots must be appreciably greater than the numberof different devices actively seeking to transmit at a given time.First, unless there are as many available backoff slots as devicesactively seeking to transmit, it will be impossible for each device tochoose a different backoff slot. Secondly, if the number of devicesactively seeking to transmit is large, and the number of backoff slotsequals only slightly exceeds the number of devices, it will be highlyimprobable for a satisfactory outcome to occur.

As one example, if the same K available backoff slots are available foreach of N devices actively seeking to transmit, then the probabilitythat the backoff slot that is chosen by a given device is not chosen byany of the other devices is (1−1/K)^(N-1). If K=r*N, and N is large,this is approximately e^(−1/r). For this probability to exceed 0.9 (sothat the probability that this device will be involved in a collision inits next transmission will be lower than 0.1), r should be at least 9.5:that is, there should be at least 9.5 times as many available backoffslots as devices actively seeking to transmit. For this probability toexceed 0.95 (so that the probability that this device will be involvedin a collision in its next transmission will be lower than 0.05), rshould be at least 19.5. This in turn causes many backoff slots to gounused, lowering overall network throughput.

Since the loss of overall network throughput due to a collision (loss ofuse of the shared medium for the duration of the longest packet that anyof the colliding devices transmits) usually exceeds the loss due abackoff slot going unused (loss of use of the shared medium for theduration of the backoff slot), it is generally desirable to make theprobability of a collision correspondingly lower, requiring an increasedratio r of available backoff slots to number of devices actively seekingto transmit, and lowering overall network throughput.

Alternatively, a common procedure used in wireless systems is for thetransmitter to send an initial short packet announcing theproposed/desired duration of the current frame exchange sequence and forthe receiver to respond declaring the wireless medium apparently freefor that duration, followed by the actual data transmission. In thisway, a collision between two or more transmitters employing thisstrategy will only cause a futilely busy wireless medium for theduration of the initial short packet, rather than the (possibly muchlonger) duration of the intended data packet. In the IEEE 802.11protocol, the initial short packet is a “Request-to-Send” (“RTS”)packet. This common strategy mitigates some of the negative effects of acollision, but at the cost of additional protocol overhead, since wherethere is no collision the protocol requires the additional full exchange(in IEEE 802.11, an RTS, followed by a short inter-frame space (“SIFS”),followed by a “Clear-to-Send” (“CTS”), followed by another SIFS), whichcomprises significant non-data-carrying overhead, which it is desirableto avoid. The method of the present invention may additional be used,without limitation, to comprise this and other sources of protocoloverhead that arise from the possibility of collisions in the baselineCSMA/CA (carrier sense multiple access with collision avoidance) system.

SUMMARY OF THE INVENTION

It is therefore an objective of the present invention to provide aprotocol for a mode of operation and/or an access method between anaccess point and a station of the wireless communication system, whichmay decrease the collision probability to improve overall networkthroughput without using a large contention window (i.e. without usingtoo many K available backoff slots), to solve the above-mentionedproblems.

According to one embodiment of the present invention, a wirelesscommunication system comprises an access point and at least one station,wherein when the access point operates in a setup mode, both the accesspoint and station establish a specific time slot for transmitting apacket from the station to the access point; and when the access pointoperates in a data mode following the setup mode, the station directlyuses the specific time slot determined in the setup mode to transmit thepacket to the access point.

According to another embodiment of the present invention, a wirelesscommunication method comprises: in a setup mode of an access point,establishing a specific time slot for transmitting a packet from astation to the access point; and in a data mode following the setup modeof the access point, directly using the specific time slot determined inthe setup mode to transmit the packet from the station to the accesspoint.

According to another embodiment of the present invention, a wirelesscommunication method of a station comprises: establishing a specifictime slot with an access point in a setup mode; and directly using thespecific time slot determined in the setup mode to transmit a packet tothe access point in a data mode.

According to another embodiment of the present invention, a wirelesscommunication method of a station comprises: operating in a setup modeto establish a specific time slot with a station in a setup mode; andoperating in a data mode following the setup mode to receive a packetfrom the station at the specific time slot determined in the setup mode.

These and other objectives of the present invention will no doubt becomeobvious to those of ordinary skill in the art after reading thefollowing detailed description of the preferred embodiment that isillustrated in the various figures and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating a wireless communication systemaccording to one embodiment of the present invention.

FIG. 2 is a diagram illustrating a setup mode of the wirelesscommunication system according to one embodiment of the presentinvention.

FIG. 3 is a diagram illustrating a data mode of the wirelesscommunication system, following the setup mode shown in FIG. 2,according to one embodiment of the present invention.

FIG. 4 is a diagram illustrating a setup mode of the wirelesscommunication system according to another embodiment of the presentinvention.

FIG. 5 is a diagram illustrating a data mode of the wirelesscommunication system, following the setup mode shown in FIG. 4,according to one embodiment of the present invention.

FIG. 6 is a diagram illustrating two executing sequences of the setupmode and the data mode according to one embodiment of the presentinvention.

FIG. 7 is a diagram illustrating the roster compression according to oneembodiment of the present invention.

FIG. 8 is a diagram illustrating a data mode according to anotherembodiment.

FIG. 9 is a diagram illustrating how to diagnose the hidden nodesaccording to one embodiment of the present invention.

FIG. 10 shows the CTS-to-self packet and the roster invocation packet.

FIG. 11 shows different time countdowns according to one embodiment ofthe present invention.

DETAILED DESCRIPTION

Please refer to FIG. 1, which is a diagram illustrating a wirelesscommunication system 100 according to one embodiment of the presentinvention. As shown in FIG. 1, the wireless communication system 100comprises an access point (AP) 110 and a plurality of stationsSTA1-STAn. In this embodiment, the AP 110 is wirelessly connected to thestations STA1-STAn, and allows the stations STA1-STAn to connect to awired network using WiFi or other related standards; and the stationsSTA1-STAn can be any client device such as notebook, laptop, smartphone,tablet or any other electronic device capable of wirelessly connectingto the AP 110.

In this embodiment, the AP 110 pre-stores one or more rosters, and eachroster has a roster number and a roster length, wherein the rosterlength is a number of time slots.

Please refer to FIG. 2, which is a diagram illustrating a setup mode ofthe wireless communication system 100 according to one embodiment of thepresent invention. Referring to FIG. 2, when the AP 110 operates in thesetup mode, the AP 110 broadcasts an initialization packet or sequenceof packets, where the initialization packet or sequence comprises aroster number, roster length and a current network allocation vector(NAV) which may be regarded as duration of the setup mode. In thefollowing description, it is assumed that the roster number is “1”, theroster length is 30, and the NAV may be 1 ms, but it is not a limitationof the present invention.

After the stations STA1-STAn receive the initialization packet orsequence, for each station, if the station wants to send a data packetto the AP 110, the station may select a random time slot within theroster length, then sends a request to send (RTS) with information ofthe selected time slot, and when the AP 110 accepts the RTS/selectedtime slot, the AP 110 marks the selected time slot as assigned and sendsa clear to send (CTS) to the station. For example, if the stations STA1and STA3 want to send data packet to the AP 110, then the station STA1may select a time slot #2 and send a RTS with the information of thetime slot #2 to the AP 110, and the AP 110 receives the RTS from thestation STA1 and determines if the time slot #2 is allowed to be used bySTA1 for transmitting data packet, and if the time slot #2 is acceptedby the AP 110, the AP 110 will mark the time slot #2 as assigned andsend the CTS to the station STA1 to confirm the time slot #2 selected bythe station STA1; and after receiving the CTS, the station STA1 alsomarks the time slot #2 as an assigned time slot to STA1. Similarly, thestation STA3 may select a time slot #8 and send a RTS with theinformation of the time slot #8 to the AP 110, and the AP 110 receivesthe RTS from the station STA3 and determines if the time slot #8 isallowed to be used by STA3 for transmitting data packet, and if the timeslot #8 is accepted by the AP 110, the AP 110 will mark the time slot #8as assigned and send the CTS to the station STA3 to confirm the timeslot #8 selected by the station STA3; and after receiving the CTS, thestation STA3 also marks the time slot #8 as an assigned time slot toSTA3.

In the embodiment shown in FIG. 2, both the stations STA1 and STA3confirm the time slots with the AP 110, therefore, each of the stationsSTA1 and STA3 is allowed to transmit a data packet to the AP 110 whenthe AP 110 operates in the data mode following the setup mode. It isnoted that in the setup mode, in this embodiment, the stations STA1 andSTA3 do not transmit any data packet to the AP 110.

Refer to FIG. 3, which is a diagram illustrating a data mode of thewireless communication system 100, following the setup mode shown inFIG. 2, according to one embodiment of the present invention. Referringto FIG. 3, when the AP 110 operates in the data mode, that is the AP 110invokes a roster (e.g. the roster number “1” mentioned above), the AP110 broadcasts an invocation signal or sequence, where the invocationsignal or sequence comprises a roster number, roster length and acurrent network allocation vector (NAV) which may be regarded as aduration of this instance of the data mode. In the followingdescription, it is assumed that the roster number is “1”, the rosterlength is 30, and the NAV may be 20 ms, but it is not a limitation ofthe present invention.

After the stations STA1 and STA3 receive the i invocation packet, eachstation directly uses the time slot selected for the roster number inthe setup mode to transmit the data packet to the AP 110. For example,after receiving the invocation packet or sequence having the informationof the roster number “1”, the station STA1 directly uses the time slot#2 selected for the roster number “1” in the setup mode to transmit thedata packet to the AP 110; and after receiving the data packet from thestation STA1, the AP 110 responds with an acknowledgement ACK (ifnecessary under the acknowledgement policy in operation). Similarly,after receiving the invocation packet or sequence having the informationof the roster number “1”, the station STA3 directly uses the time slot#8 selected for the roster number “1” in the setup mode to transmit thedata packet to the AP 110; and after receiving the data packet from thestation STA3, the AP 110 also responses an acknowledgement ACK. WhileSTA1 transmits its packet, STA3 (and all other STAs following STA1 inthis instance of the roster data phase) decodes the packet durationfield of STA1's packet and freezes its time countdown: that is, STA3(and any other STA) does not count down backoff slots for that duration.(In this way, the establishment and assignment of “time slots” for usein the data phase of the roster system may be understood as reservingtime slots out of the time that is unused by actual data transmissions:time slots that would ordinarily be used as backoff slots.)

Here and throughout the present disclosure, the use of “time slot”should be taken as equivalent to “available backoff slot”. Commonwireless protocols, including notably IEEE 802.11, include additionalinter-frame-spaces of varying lengths before backoff slots to facilitateoperations such as clear-channel-assessment (“CCA”),transmitter-to-receiver and receiver-to-transmitter turnaround time, andothers. These (with appropriately modified durations) may also beusefully used in combination with the present invention. The essentialcharacteristic of the present invention is that in the available backoffslots, there can be no collisions between devices that participate inthe disclosed protocol and that can hear each other's transmissions.

Please refer to FIG. 4, which is a diagram illustrating a setup mode ofthe wireless communication system 100 according to another embodiment ofthe present invention. Referring to FIG. 4, when the AP 110 operates inthe setup mode, the AP 110 broadcasts an initialization packet orsequence, where the initialization packet or sequence comprises a rosternumber, roster length, a current network allocation vector (NAV) whichmay be regarded as a duration of the setup mode, and a bitmap ofavailable/unavailable time slots. In the following description, it isassumed that the roster number is “2”, the roster length is 40, the NAVmay be 1 ms, and the available time slots are #10-#30, but it is not alimitation of the present invention.

After the stations STA1-STAn receive the initialization packet orsequence, for each station, if the station wants to send a data packetto the AP 110, the station may select a random unassigned time slotwithin the roster length, then send a RTS in the selected time slot, andif the AP 110 accepts the RTS/selected time slot, the AP 110 marks theused time slot as assigned and sends a CTS to the station. For example,assuming that the stations STA1-STA5 want to send data packet to the AP110, in FIG. 4, if the station STA2 sends the RTS too soon after the RTSsent by the station STA1, then the collision occurs and the AP 110 willnot receive the RTS sent by the stations STA1 and STA2. Then, thestation STA4 may select a time slot #15 and send a RTS with theinformation of the time slot #15 to the AP 110, and the AP 110 receivesthe RTS signal from the station STA4 and if STA4's request for time slot#15 is accepted by the AP 110, the AP 110 will send the CTS to thestation STA4 to confirm the time slot #15 selected by the station STA4.(The AP may choose to decline a request by a STA for a time slot for anyof a variety of reasons, including without limitation that the same STAhas already been granted enough other time slots, and that the same STAhas previously transmitted an inadequately high priority class oftraffic.) In addition, if the station STA5 sends a RTS when the NAV isexpired (i.e. over 1 ms), the AP 110 will not send the CTS to thestation STA5.

In the mode described above, the bitmap of unassigned slots may bedetermined in any way. For example, and without limitation, the AP maydetermine currently active STAs and communicate individually with eachSTA by other means to pre-assign time slots; or the AP may reserve timeslots for its own use, or the AP may choose to reserve time slots forpossible future use (for example for possible future requests for timeslots for high priority traffic), or any combination of these.

In the embodiment shown in FIG. 4, only the station STA4 establishes thetime slot with the AP 110, therefore, the station STA4 is allowed totransmit a data packet to the AP 110 when the AP 110 operates in thedata mode following the setup mode, but the stations STA1-STA3 and STA5are not allowed to transmit data packets to the AP 110. It is noted thatin the setup mode, the stations STA1-STA5 do not transmit any datapacket to the AP 110.

Refer to FIG. 5, which is a diagram illustrating a data mode of thewireless communication system 100, following the setup mode shown inFIG. 4, according to one embodiment of the present invention. In FIG. 5,when the AP 110 operates in the data mode, that is the AP 110 invokes aroster (e.g. the roster number “2” mentioned above), the AP 110broadcasts an invocation signal or sequence, where the invocation signalor sequence comprises a roster number, a roster length, a currentnetwork allocation vector (NAV) which may be regarded as a duration ofthe data mode, and a bitmap of the assigned time slots. In the followingdescription, it is assumed that the roster number is “2”, the rosterlength is 40, and the NAV may be 20 ms, but it is not a limitation ofthe present invention.

After the station STA4 receives the invoked packet having theinformation of the roster number “2”, the station STA4 directly uses thetime slot #15 selected in for the roster number “2” in the setup mode totransmit the data packet to the AP 110; and after receiving the datapacket from the station STA4, the AP 110 responses an acknowledgementACK, if required by the acknowledgement policy in use. In addition,because the stations STA1-STA3 and STA5 did not confirm any time slotwith the AP 110 in the setup mode, the stations STA1-STA3 and STA5 willnot transmit any data packet to the AP 110 in the data mode.

Briefly summarized, in the embodiments shown in FIGS. 2-5, when astation wants to send a data packet to the AP 110 within the roster modeof operation, the station needs to book a time slot of a roster of theAP 110 in the setup mode first, then the station is allowed to transmitthe data packet to the AP 110 in the roster data mode. By using theaccess method described above, the collision probability can be reducedor eliminated when many stations want to transmit data packets to the AP110.

In addition, in the embodiments shown in FIGS. 2-5, when a station booksa time slot of a roster of the AP 110 in the setup mode, the station isallowed to transmit one data packet to the AP 110 in the data mode. TheAP may invoke a roster many times: that is, the roster initializationphase may be a procedure carried out once, while the data mode may beentered many times. In this way, the overhead of the initializationphase may be amortized over many data mode phases.

In one embodiment, the data mode immediately follows the setup mode forthe same roster number; but in another embodiment, a setup mode for asecond roster number immediately follows a setup mode for a first rosternumber. For example, in FIG. 6, assume that the AP 110 has two rostershaving roster numbers “1” and “2”, the executing sequence may be setupmode for roster number “1”, data mode for roster number “1”, setup modefor roster number “2”, and data mode for roster number “2”, . . . , andso on (FIG. 6 (a)); and alternatively the executing sequence may besetup mode for roster number “1”, setup mode for roster number “2”, datamode for roster number “1”, and data mode for roster number “2”, . . . ,and so on (FIG. 6 (b)). It is noted that the executing sequence shown inFIG. 6 is for illustrative purposes only, as long as the data modefollows the setup mode for the same roster number, the executingsequence may have other arrangements. Similarly, the network protocolmay have many other modes of operation, and may intersperse uses ofroster initialization and roster data modes among unrelated modes ofoperation. These alternative designs shall fall within the scope of thepresent invention.

In one embodiment, a roster length of a roster may not be always thesame, that is the roster length can be changed or re-determined. Inaddition, a station may book many time slots in any roster, and may booktime slots of two or more rosters. These alternative designs shall fallwithin the scope of the present invention.

In the embodiments shown in FIG. 2 and FIG. 4, the stations send the RTSto the AP 110 first, and wait for the confirmation (s) of the AP 110 inthe setup mode. However, in another embodiment of the present invention,the AP 110 may actively poll or ask one or more specific stations tojoin a roster, and arrange these specific stations to have high prioritytime slot (s). This alternative design shall fall within the scope ofthe present invention.

In addition, in the embodiments shown in FIG. 2 and FIG. 4, in a setupmode, the AP 110 only broadcast the initialization packet once. However,in another embodiment, upon expiration of the NAV set in the setup mode,some stations may have assigned time slots, and the AP 110 maysubsequently send a broadcast packet which lists the same roster number,and optionally includes a bitmap or other signaling of all of the timeslots that have already been assigned in the roster. Then, the stationsthat do not have assigned time slots in the roster may choose one timeslot randomly, but avoiding the already assigned time slots.

In another embodiment, the participating STAs do not have to belong tothe same network as the AP 110 (e.g., in the IEEE 802.11 protocol, donot have to belong to the same “Basic Service Set” (“BSS”)). Suchdevices may exchange RTS and CTS packets with the AP 110 to book slotsin the roster, without exchange of data and without the need forassociation, authentication, and/or the various other setup steps thatcommonly occur prior to full data exchange. Since the roster method ofthe present invention is primarily directed at reducing protocoloverhead due to collisions or the possibility of collisions, and sincecollisions may occur between devices whether or not those devices arepart of the same network/BSS, it may be advantageous for the AP 110 topermit devices that are not a part of the AP 110's BSS to book slots inthe roster. Such slots may be used by the relevant STAs to transmit totheir own respective APs.

In the setup mode, if the time slots selected by the stations aredispersed, that is many unselected time slots are within the selectedslots, the data transmission efficiency may be lowered. To solve thisproblem, the AP 110 may compress the roster and reassign the time slotsfor the stations. In detail, refer to FIG. 7, which is a diagramillustrating the roster compression according to one embodiment of thepresent invention. As shown in FIG. 7, after receiving theinitialization packet or sequence from the AP 110, the stations STA3,STA1, STA2 and STA4 successfully book the time slots with the AP 110.Then, because the selected time slots are dispersed, the AP 110 mayreassign the time slots for the stations STA3, STA1, STA2 and STA4 tomake the stations have closer time slots. In this embodiment, thestations STA3, STA1, STA2 and STA4 are assigned to have new time slots#1-#4 for the same roster or another roster, and the AP 110 will recordthese new assigned time slots and notify the stations STA3, STA1, STA2and STA4.

In addition, if one station books a time slot in the setup mode, butdoes not transmit data packet to the AP 110 in the data mode, the AP 110may record this situation for further use. For example, if that stationfails to use its assigned slot sufficiently often, as determined by theAP 110, the AP 110 may mark the slot as not-to-be-used in a separatebitmap included in the roster invocation; then other participatingstations treat the time slot so marked as deleted from the roster duringthis invocation of the roster. For example, if another station has beenassigned time slot #5 in that roster, and notes from the rosterinvocation that time slot #3 is to be considered unused during thisinvocation of the roster, the station in this embodiment would considerit had been assigned and could use time slot #4 instead of time slot #5.This provides one way for the AP 110 to adapt the roster to currentusage patterns, in which a station that previously transmitted datastops transmitting.

In the above-mentioned embodiments, when a station wants to send a datapacket to the AP 110 within the roster mode of operation, the stationneeds to book a time slot of a roster of the AP 110 in the setup modefirst, then the station is allowed to transmit the data packet to the AP110 in the roster data mode. By using the access method described above,the collision probability can be reduced or eliminated when manystations want to transmit data packets to the AP 110. However, in asituation that two or more stations are hidden nodes to each other, thatis the two stations cannot hear each other, there may be a collisionproblem in the data mode. For example, if stations STA 1 and STA2 arehidden nodes, and the station STA1 books the time slot #5 of a roster,and the station STA2 books the time slot #6 of a roster in the setupmode; then in the data mode, when the station STA1 counts down to fiveand starts to transmit the data packet to the AP 110, because thestation STA2 cannot receive the information from the station STA1 anddoes not know that the station STA1 is transmitting data, the stationSTA2 will not freeze its time countdown and start to transmit data atthe time slot #6, causing the collision issue. Therefore, in thefollowing description, an embodiment is provided to solve this hiddennodes issue.

Refer to FIG. 8, which is a diagram illustrating a data mode accordingto another embodiment. As shown in FIG. 8, the AP 110 broadcasts aninvocation signal or sequence, where the invocation signal or sequencecomprises a roster number, roster length and a current networkallocation vector (NAV) which may be regarded as a duration of thisinstance of the data mode, a bitmap of available/unavailable time slots,and a bitmap of time slots that are required to use protection. The“bitmap of time slots that required to use protection” refers to thetime slots selected by the hidden nodes in the setup mode. For example,if the stations STA1 and STA2 are hidden nodes to each other, and thestation STA1 books the time slot #5 of a roster, and the station STA2books the time slot #6 of a roster in the setup mode, then the “bitmapof time slots that required to use protection” in the invocation signalor sequence comprises time slot #5 and time slot #6. Then, when thestation STA1 counts down to five (at time slot #5), the station STA1sends a CTS-to-self packet to reserve a duration for data transmission,and the AP 110 will immediately broadcast a signal (such as anotherCTS-to-self packet) to notify all the other stations that the durationhas been reserved for data transmission, thereby the other stations willfreeze its time countdown. By using this embodiment, the station STA2(the hidden node of the station STA1) will not transmit data in thereserved duration, and the collision issue can be prevented.

FIG. 9 is a diagram illustrating how to diagnose the hidden nodesaccording to one embodiment of the present invention, where thisembodiment can be regarded as an extra setup step after eachparticipating station booked a time slot of a roster. Refer to FIG. 9,the AP 110 broadcasting a diagnostic signal or sequence, where thediagnostic signal or sequence comprises a roster number, roster lengthand a current NAV and a bitmap of time slots to respond for theparticipating stations. When the participating stations receive thediagnostic signal or sequence, each participating station sends aresponse (e.g. RTS) at its assigned turn. Then, each participatingstation reports at its assigned turn a bitmap of RTSs it heard. Indetail, assuming that the stations STA1-STA5 send the RTSs,respectively, but the stations STA3 and STA5 report that they merelyheard RTSs of STA1-STA2 and STA4, then the AP 110 knows that thestations STA3 and STA5 are hidden nodes with respect to each other.

The above-mentioned initialization signal, invocation signal or sequencebroadcasted by the AP 110 may have one or more packets. For example, theinvocation signal may have a CTS-to-self packet and the rosterinvocation packet, where the CTS-to-self packet is used to notify thelegacy devices to reserve the NAV, and the roster invocation packetcomprises the roster number, roster length etc. as mentioned above. FIG.10 shows the CTS-to-self packet and the roster invocation packet, wherea short inter-frame spacing (SIFS) is between the CTS-to-self packet andthe roster invocation packet. In this embodiment, there is norequirement that the two packets must be sent at the same data rate: theCTS-to-self packet may for example be sent at the lowest data rate(corresponding to the longest range), while the roster invocation packet(which is addressed only to devices participating in the roster, whichis possibly a much smaller set of devices) may be sent at a higher datarate.

In addition, in the embodiments shown in FIG. 8, because when the hiddennode will send a CTS-to-self packet to reserve a duration for datatransmission before sending data at an assigned slot, therefore, thetime countdown for the other stations may be different at this assignedslot. In detail, refer to FIG. 11, if “bitmap of time slots thatrequired to use protection” sent by the AP 110 comprises time slot #3and time slot #4, then when the stations count down, the time countdownfor the slots #3 and time slot #4 comprise a CTS time, a SIFS and a slottime, and the other time slots merely comprise a slot time. This permitsall stations to allow an appropriate amount of time to elapse beforejudging a slot marked as to-be-protected as not used.

Briefly summarized, in wireless communication system of the presentinvention, when a station wants to transmit a data packet to an APwithin a roster, the station needs to confirm a time slot of a rosterwith the AP in the setup mode first, then in data mode the stationdirectly uses the time slot determined in the setup mode to transmit thepacket to the AP. By using the method described above, the collisionprobability can be reduced or (usually) eliminated when many stationswant to transmit the data packets to the AP, and a roster length of aroster can be set smaller to avoid too many unused time slots in thedata mode, and the overall throughput can be improved.

Those skilled in the art will readily observe that numerousmodifications and alterations of the device and method may be made whileretaining the teachings of the invention. Accordingly, the abovedisclosure should be construed as limited only by the metes and boundsof the appended claims.

What is claimed is:
 1. A wireless communication system under acontention-based protocol, comprising: an access point; and at least onestation, for wirelessly communicated with the access point under thecontention-based protocol; wherein the access point broadcasts aninitialization packet or sequence of packets to establish a roster, andboth the access point and the at least one station confirm a specifictime slot of a plurality of time slots in the roster for transmitting apacket from the station to the access point; and the stationsubsequently uses the specific time slot to transmit the packet to theaccess point.
 2. The wireless communication system of claim 1, whereinthe access point re-assigns another specific time slot for the at leastone station, and the station uses the another specific time slot totransmit the packet to the access point.
 3. The wireless communicationsystem of claim 2, wherein the another specific time slot is shorterthan the specific time slot.
 4. The wireless communication system ofclaim 1, wherein the at least one station comprises a plurality ofstations, and after the stations confirm a plurality of specific timeslots in the roster with the access point, the access point re-assignseach of the specific time slots for the stations to make the stationshave closer time slots.
 5. The wireless communication system of claim 1,wherein the access point broadcasts an invocation signal or sequence toinitiate data packet transmission, wherein the invocation signal orsequence comprises information of the roster number, the roster lengthand a bitmap of time slots that are required to use protection.
 6. Thewireless communication system of claim 5, wherein the bitmap of timeslots that are required to use protection comprises the time slots thatare reserved by two more hidden stations/hidden nodes.
 7. The wirelesscommunication system of claim 5, wherein when the bitmap of time slotsthat are required to use protection comprises the specific time slot,the station sends a CTS-to-self packet to reserve a duration, the accesspoint notifies all the stations that the duration is reserved, and thenthe station sends the data packet to the access point.
 8. A wirelesscommunication method of a wireless device using a contention-basedprotocol, comprising: using an initialization packet or sequence toenable an establishment of a specific time slot of a roster;establishing/confirming the specific time slot of a plurality of timeslots in the roster via a wireless medium; and using the specific timeslot to transmit a packet via the wireless medium; wherein the step ofusing the specific time slot to transmit the packet comprises: receivingan invocation signal or sequence to initiate data packet transmission,wherein the invocation signal or sequence comprises information of aroster number, a roster length and a bitmap of time slots that arerequired to use protection.
 9. The wireless communication method ofclaim 8, further comprising: re-establishing/re-confirming anotherspecific time slot; and using the another specific time slot to transmitthe packet via the wireless medium.
 10. The wireless communicationmethod of claim 9, wherein the another specific time slot is shorterthan the specific time slot.
 11. The wireless communication method ofclaim 8, further comprising: establishing/confirming a plurality ofspecific time slot of the plurality of time slots in the roster via thewireless medium; re-establishing/re-confirming each of the specific timeslots to make the reassigned time slots are closer to each other. 12.The wireless communication method of claim 8, wherein the step of usingthe specific time slot to transmit the packet further comprises: whenthe bitmap of time slots that are required to use protection comprisesthe specific time slot, sending a CTS-to-self packet to reserve aduration, notifying all stations that the duration is reserved, and thensending the packet to an access point.
 13. The wireless communicationmethod of claim 12, wherein the bitmap of time slots that are requiredto use protection comprises the time slots that are reserved by two morehidden stations/hidden nodes.
 14. A wireless device using acontention-based protocol, the wireless device comprising: a circuitryconfigured to use an initialization packet or sequence to enable anestablishment of a specific time slot of a roster confirm the specifictime slot of a plurality of time slots in the roster via a wirelessmedium; utilize the confirmed specific time slot to transmit a packetvia the wireless medium; and receive an invocation signal or sequence toinitiate data packet transmission, wherein the invocation signal orsequence comprises information of a roster number, a roster length and abitmap of time slots that are required to use protection.
 15. Thewireless device of claim 14, wherein the circuitry is further configuredto: re-confirm another specific time slot; and use the another specifictime slot to transmit the packet via the wireless medium.
 16. Thewireless device of claim 15, wherein the another specific time slot isshorter than the specific time slot.
 17. The wireless device of claim14, wherein the circuitry is further configured to: confirm a pluralityof specific time slot of the plurality of time slots in the roster viathe wireless medium; re-confirm each of the specific time slots to makethe reassigned time slots are closer to each other.
 18. The wirelessdevice of claim 14, wherein the circuitry is further configured to: whenthe bitmap of time slots that are required to use protection comprisesthe specific time slot, send a CTS-to-self packet to reserve a duration,notify all stations that the duration is reserved, and then send thepacket to an access point.
 19. The wireless device of claim 18, whereinthe bitmap of time slots that are required to use protection comprisesthe time slots that are reserved by two more hidden stations/hiddennodes.